An ultrasonic diagnostic apparatus is used to monitor an internal tissue of a subject by irradiating him or her with an ultrasonic wave and analyzing the information contained in its echo signal. For example, a conventional ultrasonic diagnostic apparatus that has been used extensively converts the intensity of an echo signal into its associated pixel luminance, thereby presenting the subject's structure as a tomographic image. In this manner, the internal structure of the subject can be known. The ultrasonic diagnostic apparatus can be used to make a noninvasive checkup on an internal tissue of a subject, and therefore, is now an indispensable device at any clinical spot along with X-ray CT and MRI.
Recently, the number of people suffering from atherosclerosis has been on the rise and measurement of CIMT with B-mode ultrasonic has been carried out more and more often using an ultrasonic diagnostic apparatus to diagnose atherosclerosis. As the atherosclerosis advances, the vascular wall becomes increasingly thick and the blood vessel becomes narrower and narrower. That is why the atherosclerosis can be diagnosed by measuring the thickness of the vascular wall. It is known that the carotid artery has a three-layer structure consisting of intima, media and adventitia that are stacked in this order. For identifying and quantifying atherosclerosis, the combined thickness of the intima and the media at the carotid artery (or intima-media thickness, which will be abbreviated herein as “IMT”) is measured and used as an index to atherosclerosis. According to Non-Patent Document No. 1, if the IMT is 1.1 mm or more, the carotid is determined to have abnormally thickened.
In the prior art, the IMT is manually measured. Specifically, the operator locates the intima, media and adventitia on a tomographic image that has been generated by the method described above, and measures the thickness with a length measuring function on the tomographic image, which is usually provided for any ultrasonic diagnostic apparatus.
Also, recently, thanks to the development of electronics technologies, the precision of measurements of ultrasonic diagnostic apparatuses could be improved by leaps and bounds. As a result, as disclosed in Patent Document No. 1, some people are attempting recently to track the movement of a subject's tissue more precisely and evaluate the strain, modulus of elasticity or any other physical (attribute) property of the subject's tissue (among other things, an arterial vascular wall) mainly by analyzing the phase of the reflected wave.
Even in calculating the modulus of elasticity of a vascular wall, the thickness of the vascular wall also needs to be measured. That is why the combined thickness of a vascular wall, including the intima, media and adventitia, is manually measured with the length measuring function on a tomographic image as described above.                Patent Document No. 1: Japanese Patent Application Laid-Open Publication No. 10-5226        Non-Patent Document No. 1: Hiroshi Furuhata, Carotid Echo, Vector Core Inc., 2004, ISBN 4-938372-88-6        Non-Patent Document No. 2: S. Golemati, et al., Ultrasonic Med. Biol. Vol. 29, pp. 387-399, 2003        Non-Patent Document No. 3: J. Bang et al., Ultrasonic Med. Biol. Vol. 29, pp. 967-976, 2003        Non-Patent Document No. 4: M. Cinthio, et al., IEEE Trans. Ultrason. Ferroelect. Freq. Contr. Vol. 52, pp. 1300-1311, 2005        